As the information age has evolved and Internet usage has expanded, data transmission capacity has become increasingly important. At present, much of the data transmission load rests atop fiber optic networks. These fiber optic networks provide the backbone for many, if not most, networked data transmission systems.
Fiber optic networks use glass or plastic threads (i.e., fibers) to transmit data. A typical fiber optic cable consists of a bundle of these glass or plastic threads, each of which is capable of transmitting messages modulated onto light waves.
In recent years, the data transmission capacity of fiber optic cables has increased as a result of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). With WDM and DWDM systems, signals assigned to different wavelengths are combined into a single signal for transmission over a single line or medium.
In operation, a typical DWDM system may modulate each of several data streams onto a different part of the light spectrum. For example, one data stream may have an assigned wavelength of 1543 nanometers (nm) and the next data stream may have an assigned wavelength of 1543.8 nm. In most cases, the required spacing between assigned wavelengths is set by the International Telecommunication Union (ITU). Typical spacings include 0.4 nm and 0.8 nm.
The process of multiplexing has a counterpart, demultiplexing. Demultiplexing typically refers to the separation of a transmission coming through a single line or medium back into its constituent signals for further processing. Both multiplexing and demultiplexing are integral to the operation of a DWDM system.
The actual processes of multiplexing and demultiplexing within DWDM systems have conventionally involved very expensive and difficult to manufacture devices. Many, if not most, conventional DWDM devices are integrated optics devices that require a photolithography manufacturing step, which may account for a portion of the high cost associated with such devices.
Apart from being expensive to manufacture, conventional DWDM devices have operational shortcomings. For example, DWDM devices often have a high sensitivity to light polarization. Frequently referred to as polarization dependent loss (PDL), this high sensitivity to light polarization lessens the overall effectiveness and efficiency of conventional DWDM devices.